The BepiColombo Spacecraft can’t tolerate to absorb a major fraction of the off-axis sunlight through larger payload apertures. Fortunately, there are solutions to design baffles such that they reflect the incoming radiation back through the front aperture rather than absorbing it. A Design Study, sponsored by ESA and performed by Contraves Space together with SAGEM Défense Securité, has analysed the potential of various solutions and assessed the options to manufacture them. The selected configuration has been analysed in detail for the optical, mechanical and thermal performance as well as the impact on mass and power dissipation. The size of the baffle was adapted to the needs of the BepiColombo Laser Altimeter (BELA) payload.

High precision space optics, such as spectrometers, relay optics, and filters, require ultra stable, lightweight platforms. These equipped platforms have on one side to survive the launch loads, on the other side they have to maintain their stability also under the varying thermal loads occurring in space. Typically such platforms have their equipment (prisms, etalons, beam expanders, etc.) mounted by means of classical bonding, hydro-catalytic bonding or optical contacting. Therefore such an optical bench requires to provide an excellent flatness, minimal roughness and is usually made of the same material as the equipment it carries (glass, glass ceramics).

For space systems, mass is a big penalty, therefore such optical platforms are in most cases light weighted by means of machining features (i.e. pockets). Besides of being not extremely mass efficient, such pockets reduce the load carrying capability of the base material significantly.

The challenge for Oerlikon Space, in this context, was to develop, qualify and deliver such optical benches, providing a substantial mass reduction compared to actual light weighted systems, while maintaining most of the full load carrying capacity of the base material.

Additionally such a substrate can find an attractive application for mirror substrates. The results of the first development and of the first test results will be presented.

RUAG Space, together with THALES SESO, initiated the development of light weighted sandwich structures
for optical applications, already some years ago. The results of the use of this type of structure applied for
optical benches and the first outlook for their use as mirror substrates were published in previous papers. This
paper is going to present the results of the polishing activities performed on a 650 mm diameter mirror
manufactured with Zerodur face sheets and a low density aluminum core. This substrate showed a mass
density of 15 kg/m2. The excellent optical quality achieved proves the suitability of this technology for several
applications, in particular for scanning mirrors for space and possibly for moveable mirror in ground based
astronomical telescopes.
With the emerging need for extremely high flatness under thermal loads (radius of curvature < 400km)
activities have been initiated to identify materials for the core of the substrate closer matching the extremely
low CTE offered by materials like Zerodur. The progresses made in this field are presented and an outlook for
future activities is provided.

RUAG Space developed, manufactured and demonstrated an afocal mirror telescope for space applications. The
telescope is part of a Laser Communication Terminal (LCT) for GEO and LEO satellites. The design is off-axis and free
of central obscuration. Optical interfaces are provided by pupils outside the telescope towards space (ø=135 mm) and
towards the payload (ø=12.5 mm). The magnification is Γ=-10.8. The main characteristics are a WFE of ≤35nm,
transmission <96%, low extinction ratio of linear and circular polarization, low stray light and low mass. The
performance stability was demonstrated under various environments including vibrations, shock and thermal-vacuum up
to 55°C. These properties enable a broad use, not limited to space. The layout is composed of four mirrors (Zerodur and
Fused Silica) integrated in a nearly zero expansion Carbon Fibre (CFRP) structure. A detailed characterisation and
advanced understanding of the CFRP represents a main achievement. The water absorption of CFRP in air causes elastic
distortions of the structure until saturation. Certain optical performances are affected by this phenomenon which has to
be considered when testing the system in thermal-vacuum environment. These effects were characterised and precompensated
during integration in order to tailor the performance to the in-orbit conditions. The stability of the
performances confirmed the selection of the CFRP as nearly-zero CTE material. Combined effects of moisture release
and thermo-elastic distortions under thermal-vacuum loads were detected. The optical performances verification was
then consequently and successfully tailored in order to distinguish these effects and prove the telescope stability under
thermal-vacuum environment.

High precision space optics requires ultra stable lightweight platforms, which have to survive the launch loads and to
maintain their stability under space environment. Such benches require a high planarity, small roughness and are usually made of the same material as the carried equipment. For space systems mass is a penalty, thus optical platforms are lightweighted by machining features. Besides of being not too mass efficient, this reduces significantly the load carrying capability of the base material. The challenge for us was to develop, qualify and deliver optical benches, providing a high mass reduction, while maintaining the load carrying capacity of the base material. Such an optical bench has been developed and built by Oerlikon Space and was equipped together with SESO with the high performing optical components for the flight model of the Aladin Mie Spectrometer for Astrium Toulouse. All tests
have proven the high strength and stability of the adopted concept.

To ensure the performance of optical systems for space applications, the design of the mounts for the optical elements
and the choice of materials are crucial. Beside this also the applied bonding techniques are playing a major role. The
alignment of the optical elements must remain after the loads of the launch phase and in the thermal environment of the
satellite. We present our achievements in alignment accuracy and stability during assembly and integration of optical
systems for space applications in the case of two very different examples:
In the first example we bonded prisms to a baseplate using a radiation activated optical adhesive. The achieved
alignment accuracy was better than 3". In the second example we bonded Zerodur mirrors with diameters up to 150 mm
and mass of 1 kg to Invar mounting frames using a slow curing two-component adhesive. Here the achieved alignment
accuracy was in the order of 10". Thanks to our sophisticated bonding techniques and specially designed mounts and
bonding jigs, these alignments were preserved during environmental tests like thermal cycling and vibration tests.

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